In linear beam electron tubes the source of electrons is a cathode, which, to achieve low electron emission densities, is usually larger than the desired beam diameter. Electrons emitted by the cathode are acted upon by a set of electrodes with voltages impressed thereon which causes the electrodes to accelerate and optically focus the electrons to the desired beam size. The magnetic focusing field than constrains the beam and prevents it from spreading. The magnetic focusing field can be produced either by electromagnets, permanent magnets, or a combination of the two.
There are two preferred systems for focusing linear electron beam devices. One system is called Brillouin focusing in which shielding is used to prevent leakage of any of the magnetic focusing field into the cathode and beam-forming region. Nearly all the desired magnetic focusing field is introduced abruptly at or near the point the beam reaches its desired diameter.
A second focusing system is termed “confined-flow” focusing. In this system a magnetic focusing field is “leaked” into the cathode and beam-forming region in a controlled manner such that the magnetic field force lines are essentially aligned with the optical electron trajectories. In this case the magnetic focusing field approaches its full value near the point where the beam reaches its desired diameter.
Of these two focusing systems, Brillouin focusing is the weaker of the two because of the necessity to match the magnitude of the focusing field to the electron energy to properly focus the beam. The result is weaker focusing and a beam more susceptible to defocusing effects caused by rf-field interactions with the beam. Confined-flow focusing, by contrast, uses focusing fields that typically are at least two times stronger than the Brillouin focusing fields for the same device. Thus Brillouin focusing, which is a simpler system, is generally used for lower power applications, and confined-flow focusing is used almost exclusively with higher power devices.
Both of these focusing systems, when appropriately applied, work well for focusing devices with a single linear beam. In such cases the beam axis and the focusing field axis can be aligned to achieve radial and azimuthal symmetry, and the design problem becomes essentially one-dimensional—only the magnitude of the axial magnetic field must be controlled.
It has long been recognized that the designers of electron devices with multiple linear beams face a difficult 3-dimensional design problem. Much of this problem has been avoided in many of the existing multiple-beam devices by using Brillouin focusing. However, this has limited the power levels achieved. It is a purpose of this invention to teach a novel method of applying confined-flow focusing to multiple beam devices, thus opening the way for new and higher power multiple beam devices.
The electron beam is focused by a magnetic field so as to produce a beam in the RF interaction circuit of the device having a somewhat smaller diameter than the inside (or minimum) diameter of the circuit and with minimal or low scalloping. To accomplish this with a convergent electron beam (due to the cathode or emitter being of larger diameter than the desired diameter in the RF interaction circuit), an appropriate magnetic circuit (including permanent magnets and/or a solenoid) is used to shape the magnetic field along the length of the device. In the case of multiple beams, however, the beam axes are not coincident with the axis of the magnetic circuit. In such a case, extra effort must be made in the design phase to assure adequate symmetry of the magnetic focusing field within the electron beams to avoid beam interception on the RF interaction circuit. This is particularly critical for confined-flow focused beams for which a magnetic field is present in the gun and cathode region of the device.
Confined-flow magnetic focused multiple beam devices are known. In such devices, the asymmetric magnetic field (with respect to the electron beam) typically causes the individual electron beams to twist or corkscrew in a helical pattern about the axis of the electron beam as they progress from the cathode toward the anode. Devices employing confined-flow magnetic focusing therefore must take into account this twisting. This is often accomplished by placing a series of apertures along the anticipated path of the beam with the apertures arranged so that the beam is (hopefully) centered on the apertures' respective longitudinal axes. The apertures need to be spatially offset from location to location along the beam(s) so as to properly intercept the beam(s).
Some designs for multi-beam devices cluster the cathode emitters near the longitudinal axis of the device so that the individual beam axes are disposed near the axis. This technique reduces, but does not entirely eliminate, the twisting of the beam. Such devices typically have performance limitations, including device life and operating voltage limitations, that result from space restrictions caused by placing the individual beams near the longitudinal axis of the device.
Various methods for achieving magnetic field symmetry equalization have been employed. These include using individual cathode coils to shape the magnetic field, an approach which can be difficult and complex to implement. Bulky and heavy iron field-shaping elements have been suggested for use in this application together with employing displacements in the position of the beam apertures, as described above, in the gun magnetic polepiece, to achieve magnetic field symmetry.
A problem with prior confined-flow multi-beam devices that employ offset pole-piece apertures to aid in focusing the beams is that the apertures, which are fixed in position, will be properly positioned for only one set of operating conditions because the amount of twist depends upon beam current and voltage and magnetic field strength. If the device is operated outside of the specified designed-in conditions, the beam will intersect with portions of plates through which the apertures are placed at places other than the apertures resulting in damage to the device and non-optimal operation, or the beam will pass off-center through the apertures (rather than hitting the polepiece) and thereby induce further field asymmetry and therefore suffer greater beam twist.
Confined-flow multi-beam devices with beams disposed near the device axis additionally suffer from performance limitations that result from space restrictions within the device. These limitations include shorter device life due to higher operating cathode current density, operating voltage limitations due to higher electrode voltage gradients, and mechanical and thermal design challenges imposed by the requirement to work within a restricted space.